Some diseases may be communicated by the transmission of body fluids between infected and uninfected individuals or contact with infected materials. Prevention of such infection depends upon breaking the chain of transmission.
In the practice of dentistry and related professions, procedures such as root canals, installing crown and bridge work, implanting inlays, cementing orthodontic appliances and other procedures which require movement of a tool from the patient's mouth to the mixing slab to acquire mixed cement or medicants, create the opportunity for the contamination of the mixing slab with any infection of the patient. While some infections are short-lived and pose no significant health risk, others are persistent and are not readily killed by disinfection procedures.
Present practices in a dental or related practice follow either a disinfection or a sterilization approach. In the chemical disinfection approach, the implements used to treat one patient are washed and then disinfected with a suitable disinfectant, such as glutaraldehyde or glutaraldehyde oxide, by soaking the tools and implements in the disinfectant for an extended period of time. If the soaking of the tools and implements is prolonged for a sufficiently long period, for example, six to ten hours, the disinfectant soaking is tantamount to sterilization.
The alternative to the chemical disinfectant soak is sterilization with heat, heat and steam, or heat and chemicals. Sterilizing with heat, with or without steam or chemicals, is much more rapid and permits the tools and implements to be reused more frequently. This reduces the number of sets of tools necessary to establish a dental or orthodontic practice, particularly when the practice has several work stations and where multiple patients are served simultaneously.
Typically, an autoclave is used to heat the washed dental tools and implements to an elevated temperature and to hold that temperature for a short time in comparison to the time for soaking disinfection.
The autoclave heats the contents rapidly to a temperature of about 250-275 degrees F. (121-134 degrees C.) for a period of 15-40 minutes, then the contents are allowed to dry. With dry heat, the tools are maintained at about 320 degrees F. (160 degrees C.) for two hours. In a Chemclave the conditions for sterilization are to maintain the objects as 270 degrees F. (132 degrees C.) for twenty minutes.
Glass plate dental mixing slabs are presently very widely used in most practices and are the only reusable slabs available from most dental supply houses. When these glass slabs are heated in an autoclave, dry heat oven or Chemclave, the exterior of the slab is heated much more rapidly than the interior of the typically 1/2 to 3/4 inch thick slab of glass. The rapid heating of the slab creates internal stresses in the glass mixing slab and the slab may crack or shatter.
Similarly, if the slab survives the heating portion of the cycle, the rate at which glass cools is much slower than the rate of the steel of which the other items used in the practice are fabrocated and, therefore, the glass mixing slab is still very hot when the autoclave cycle is complete. When the door of the autoclave is opened, the ambient air of the room will replace the air of the autoclave and the thermal shock of the cool air may cause the slab to crack or shatter, as the exterior glass attempts to contract more rapidly than the interior portions.
Accordingly, the use of glass slabs requires that a dental office have large numbers of the slabs available and use long term soaking to sterilize the slabs, or alternatively, the use of an oven or autoclave which has a very long, slow heat up and cool-down cycle to prevent damage to the glass mixing slab. This also necessitates a separate heating device for the slabs, or the other tools would not be available for repeated use as rapidly as possible.
Further the cements and adhesives used to attach crowns, bridges, inlays, orthodontic appliances or other items to the teeth, often are materials that create an exothermic reaction when mixed. The cement and adhesive materials are also of a heat-sensitive nature, setting faster in a warm environment and slower in a colder environment.
When the cement is mixed, it is desired to keep the temperature of the mix at a temperature well below the set temperature allowing the item to be cemented, worked, placed appropriately, and positioned in place. Allowing the mix to overly warm during either the mixing or the time when it is resident on the slab prior to its use, will result in set-up before it can be used, necessitating mixing another batch of cement.
Pot life, the time the cement may be worked and used, may be extended by cooling. With glass slabs, this cooling is accomplished by refrigerating the slab prior to use. The mass of the glass slab is sufficient to act as a substantial heat sink to draw the exothermically produced heat from the mix and maintain the cement mix below the set temperature. When a glass slab is sterilized by heat, the slab must be cooled to approximately room temperature before the slab can be transferred to a refrigerator; otherwise, the slab will crack in the cold air of a refrigerator.
Some dental supply houses sell plastic mixing slabs. When the plastic slabs are heated, they will tend to soften and potentially become misshapen. Further the plastic material is much more likely to be cut, scratched or deformed in use, creating sites for contamination and residual cement to be trapped.
With a higher emphasis on sterilization of dental implements, it is less practical to use glass or plastic mixing slabs in a sterile dental office environment. To only disinfect the slab defeats the sterilization of the remainder of the tools and implements that come in contact with the glass slab.
U.S. Pat. No. 1,993,450 to Lowry, describes a thin plate of chrome/nickel alloy which is supported by a wood base. The mixing plate may be cleaned and sterilized but the wood base may not be heat sterilized readily, since the repeated heating and cooling of the wood would cause the wood to unduly dry and crack and be unserviceable.
U.S. Pat. No. 1,980,533 suggests the benefits of cooling, but uses a porous ceramic material to hold water for evaporative cooling of the ceramic slab and, thence, the cement mix. This approach is afflicted with the same short comings as a glass slab, since the glazed surface is essentially similar to glass. Further the porous nature of the device provides crevices and openings in which contaminants may reside. If evaporative cooling is used, then sterile water or alcohol must be used.
U.S. Pat. No. 1,137,482 to Hanly suggests an approach to sterile environments, but with no heat sinking capability, multiple cement mixes are required for an extended procedure, or the pot life of the cement is very limited.
U.S. Pat. Nos. 434,737 to Teal and 1,278,153 to Jefferies both disclose glass mixing slabs of varying configurations.
U.S. Pat. No. 1,660,493 to Proctor discloses a pallet of a somewhat similar shape. The Proctor pallet is described as usable for the manufacture of bricks, blocks and the like. The pallet is described as having exceptionally sharp edges and the process described by Proctor is adapted to produce such sharp edges. Sharp edges would be a hazard to the dentist or dental assistant and would open the possibility of cuts, from which the infection may be transmitted or the infection from the patient may be acquired by the dental worker through such cuts. In addition the inside corner of a sharp edged slab would be difficult to clean and thus difficult or impossible to sterilize adequately.